3D Mesh doesn't generate

Dear colleagues,

 I am facing a problem when generating a mesh for a simple steady state dam flux 3D analysis in Plaxis Groundwater. I am using Designer to generate a model volume and then importing it to Groundwater.

 The model is built with three surfaces named "Dam", "Topography" and "Basis" (upper to lower). Every time I use Dam and Topography surfaces together to create a model, Plaxis mesh generation crashes, regardless any region I use. However, when a volume is created not using both surfaces together, it works.  It is important to say that while using Designer, no error warning appears.

 I've already followed the steps presented in the Tutorial Manual, i. e. fixing intersections and merging surfaces, but nothing appears to work.

Has anyone been through a similar situation and can give a hand with it?

Thanks in advance.